Bladder-based cuff for measuring physiological parameters and method of measuring physiological parameter using same

ABSTRACT

A cuff for measuring volume and change in volume of a body appendage includes a hollow, rigid tube having an inner surface; and a bladder having an inner surface and an outer surface, the ends of the bladder being sealed to the ends of the tube to create an enclosed internal volume between the inner surface of the bladder and the inner surface of the tube and an external volume defined by the outer surface of the bladder and surrounded by the internal volume, the bladder having a normal, relaxed state, in which the internal volume is filled with a fluid and a retracted state in which the fluid is evacuated from the internal volume. Two stiffener ribs placed on the inner surface of the bladder, parallel to each other and to the lengthwise axis of the tube at diametrically opposite positions. A plurality of emitters and detectors arranged in a linear array are embedded in one of the ribs, so as to emit and detect light through the bladder. A fluid port extending through the tube and communicating with the internal volume, through which the internal volume can be filled with or emptied of the fluid.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application is a continuation of application Ser. No.10/695,441, filed Oct. 29, 2003, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The invention relates to inflatable cuff devices. More specifically, itrelates to inflatable cuff devices for measuring volume and/or thechange in volume with respect to some physiological parameter of a bodyappendage, and a method of measuring physiological parameters using aninflatable cuff device.

BACKGROUND OF THE INVENTION

Inflatable cuff devices are used in a variety of applications to measuresuch parameters as volume, change in volume, pressure, response topressure, occlusion, and response to occlusion. See, for example, U.S.Pat. Nos. 4,747,415 and 5,692,520 to Lavoisier. These measurements areused widely in, but not limited to, the medical community to assayphysiological parameters. Examples of such physiological parameters areblood pressure, cardiac cycle (plythesmography), blood flow, and pulserate, to name a few. In each case, a bladder-based cuff device is placedaround or in contact with the object to be measured (usually a bodyappendage such as an arm or a fingertip) and a gas, generally air, ispumped into the bladder, causing the bladder to inflate and thus reducethe inner diameter of the cuff. It should be noted that media other thanair can be used; however, alternative media such as liquids have thedisadvantage of being essentially non-compressible. The reduction of theinner diameter of the cuff results in an excess of bladder materialbeing compressed into the area where the cuff engages the object to bemeasured. This excess of material creates folds, bends, wrinkles, andvoids at the cuff/object interface. Depending upon the nature of themeasurement, these irregularities generally result in a source ofmeasurement error.

Also, there is a need for a device that accurately measures both thevolume and the change in volume of a body appendage as blood or otherfluids (such as water) pulse in and out or as fluid accumulates to causeswelling. For a device in which measurements of additional physiologicalparameters are made using optical emitters and detectors, the devicepreferably also holds the emitters and detectors against the bodyappendage in a predictable manner, simultaneous with measuring thevolume or the volume changes in the body appendage. In holding thecomponents against the body appendage, the bladder must not allow anyair gaps or incorrect alignment of the emitters or detectors. Thebladder must also be able to conform to the many different profiles ofbody appendages, while still allowing for accurate volume and opticalmeasurements.

The prior art includes many devices in which a bladder inflates againstthe finger. Examples of such prior art are disclosed in U.S. Pat. Nos.4,202,347 and 4,331,155 to Sacks, U.S. Pat. No. 5,025,793 to Richley etal., and U.S. Pat. No. 5,218,966 to Yamasawa. In each case, as thebladder inflates against the finger, gaps are caused by the folding ofthe bladder material. In addition, the bladders do not conform in apredictable way to each of the different shapes of fingers. Thesevariances cause any measurements made by the bladders to beunpredictable and prone to inaccuracies. Through experimentation, wehave concluded that it is not possible to build a bladder that simplyinflates from a static, un-inflated position to a variety of bodyappendage shapes and sizes without having gaps, folds, bends, orwrinkles. Thus, the prior art devices inherently introduce errors intothe measurements.

It is to the solution of these and other problems that the presentinvention is directed.

BRIEF SUMMARY OF THE INVENTION

It is therefore a primary object of the present invention to provide abladder-based cuff device for measuring physiological parameters of anappendage that conforms to any known shape and size of appendage withoutfolds, bends, wrinkles, and voids at the cuff/appendage interface, so asto insure that the measurements are accurate.

It is another object of the present invention to provide a bladder-basedcuff device that accurately measures both the volume and the change involume of a body appendage as blood or other fluids (such as water)pulse in and out or as fluid accumulates to cause swelling.

It is still another object of the present invention to provide abladder-based cuff device that conforms to any known body appendageshape and size without gaps and insures accurate and predictableplacement of the emitters and detectors.

These and other objects of the present invention are achieved by theprovision of a cuff for measuring physiological parameters of anappendage wherein the cuff includes a hollow, rigid tube having an innersurface and opposed ends; and a bladder having an inner surface, anouter surface, and opposed ends. The ends of the bladder are sealed tothe ends of the tube to create an enclosed internal volume between theinner surface of the bladder and the inner surface of the tube and anexternal volume defined by the outer surface of the bladder andsurrounded by the internal volume. The bladder has a normal, relaxedstate, in which the internal volume is filled with a fluid and aretracted state in which the fluid is evacuated from the internalvolume. Two stiffener ribs are placed on the inner surface of thebladder, parallel to each other and to the lengthwise axis of the tubeat diametrically opposite positions. A plurality of emitters anddetectors are arranged in a linear array are embedded in one of theribs, so as to emit and detect light through the bladder. A fluid portextends through the tube and communicates with the internal volume,through which the internal volume can be filled with or emptied of thefluid.

In one aspect of the invention, the bladder is tubular in shape, withthe ends of the bladder overlapping the ends of the tube, and whereinthe thickness of the bladder is greater where the ends of the bladderoverlap the ends of the tube.

In another aspect of the invention, the bladder, in its normal, relaxedstate, has an inside diameter smaller than the diameter of the smallestof the type of appendage to be measured, and is made of a material, suchas a thin wall rubber or a thin wall silicone rubber, that allows theinside diameter to stretch at least to the diameter of the largest ofthe type of appendage to be measured, and return back to its originalsize without deforming.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a bladder-based cuff in accordancewith the present invention.

FIG. 2 is a cross-sectional view taken along line 2-2 of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIGS. 1 and 2, a bladder-based cuff 10 is shown inaccordance with the present invention, comprising a hollow, rigid tube20 having an inner surface 20 a and opposed ends 20 b, and a bladder 30having an inner surface 30 a, outer surface 30 b, opposed ends 30 c, anda center portion 30 d between the opposed ends 30 c, the ends 30 c ofthe bladder 30 being sealed to the ends 20 b of the tube 20 to create anenclosed internal volume 40 a between the inner surface 30 a of thebladder 30 and the inner surface 20 a of the tube 20 and an openexternal volume 40 b defined by the outer surface 30 a of the bladder 30and surrounded by the enclosed internal volume 40 a. Preferably, thebladder 30 and the enclosed internal volume 40 a are both tubular inshape, while the open external volume 40 b is cylindrical, with thebladder 30, the enclosed internal volume 40 a, and the open externalvolume 40 b all having a common longitudinal axis that is collinear withthe longitudinal axis of the tube 20.

The bladder 30 has a normal, relaxed state, in which the enclosedinternal volume 40 a is filled with a fluid, preferably air or anothergas, and a retracted state, in which the fluid is evacuated from theinternal volume 40 a. In its relaxed state, the bladder 30 does notprovide sufficient room for an appendage to be inserted into theexternal volume 40 b; while in its retracted state, the bladder 30 doesprovide sufficient room for the appendage to be inserted into theexternal volume 40 b. The appendage can be a finger (more specifically,a fingertip), an arm, or any other appendage having one or morephysiological parameters normally measured by an inflatable cuff device.

A plurality of emitters 50 a and detectors 50 b are positioned withinthe interior of the tube 20, preferably inside the enclosed internalvolume 40 a, as discussed in greater detail hereinafter. Preferably, theemitters 50 a and detectors 50 b are positioned in a linear fashionparallel to the longitudinal axis of the tube 20, with the emitters 50 abeing side-by-side and the detectors 50 b being side-by-side. In theembodiment shown in FIGS. 1 and 2, there are two emitters 50 a and fourdetectors 50 b. The emitters 50 a may generate and emit eithermonochromatic light or multi-wavelength light, and each emitter 50 a caninclude a variety of different types of light sources, includingmultiple discrete LEDs, multiple discrete laser diodes, and multipleemitting sources combined and coupled into multi-mode fiber optics, asexamples. The number of emitters 50 a, or more specifically, the numberof wavelengths employed by the cuff 10, is governed by the number ofparameters being measured other than volume or change in volume, as adifferent wavelength is required for each parameter. The number ofdetectors 50 b is determined by the need for spatial differentiation.The factors governing the number of emitters 50 a and detectors 50 b aredescribed in U.S. Pat. No. 6,181,958, which is incorporated herein byreference in its entirety.

A fluid port 60 extends through the tube 20 and communicates with theenclosed internal volume 40 a, by which the internal volume 40 a can befilled with or emptied of the fluid. The port 60 is attachable to means(not shown) for emptying fluid from the internal volume 40 a and fillingit with the fluid. This means can comprise a manifold connected to thefluid port 60 via a valve in the manifold and a pump to move the fluidinto and out of the manifold, a syringe and a tube connecting thesyringe to the fluid port 60, or any other mechanism designed to movefluids of any type. To allow the appendage to be inserted into theexternal volume 40 b, the mechanism draws the fluid from the enclosedinternal volume 40 a, causing the bladder 30 to assume its retractedstate (thus minimizing the enclosed internal 40 a and maximizing theexternal volume 40 b). Once the appendage has been placed into theexternal volume 40 b, the mechanism releases the fluid back into theenclosed internal volume 40 a so that the bladder 30 assumes a state inwhich it conforms to the contours of the appendage.

In order for the bladder 30 to conform to any shape of appendage,without gaps, it is necessary for the bladder 30, in its normal, relaxedstate, to have an inside diameter smaller than the smallest of the typeof appendage to be measured. To achieve this goal, the bladder 30 ismade of a material that allows the inside diameter to stretch at leastto the diameter of the largest of the type of appendage to be measured.When the fluid is evacuated from the enclosed internal volume 40 a, avacuum is created and the bladder 30 stretches, causing the diameters ofthe bladder 30 and the external volume 40 b to increase until thebladder 30 conforms to the inner surface 20 a of the tube 20, thusbecoming larger than the largest of the type of appendage to be tested.When the fluid is returned to the enclosed internal volume 40 a and thevacuum is released with an appendage inside the external volume 40 b,the bladder 30 attempts to retract to its natural state and conforms tothe appendage without any wrinkles or gaps.

The selection of the bladder material is very important. The materialmust be very soft and elastic, and must be able to stretch approximatelytwo to three times its original diameter and return back to the startingsize without deforming. In addition, the bladder material must besufficiently impermeable to the fluid so as to minimize leakage,regardless of the amount the bladder 30 is stretched. The bladdermaterial also must be of a type that can achieve a stable state withrespect to its elasticity after it has been stretched. Due to theseconsiderations, materials that are suitable for the bladder 30 include,but are not limited to, a rubber or a silicone rubber.

In addition, for practical purposes, the vacuum necessary to stretch thebladder 30 must be kept low to minimize the amount of force necessary toexpand the bladder 30 so as to conform to the inner surface 20 a. If thebladder 30 is made of a rubber or a silicone rubber material, then tomeet the requirement that the bladder 30 be able to stretch toapproximately two to three times its original diameter using a lowvacuum, the bladder 30 must have a thin wall. However, the requirementof a thin wall raises another consideration. Thin wall rubbers andsilicone rubbers are permeable. Because in some appendages, particularlya finger, the volume change is very, very small as blood or other fluidspulse in and out or as fluid accumulates to cause swelling, the changein pressure is also very small. As a result, when the change in pressuredue to the pulsing or swelling of the body appendage is measured, it isalso possible to measure the pressure loss in the bladder 30 due to theloss of fluid due to leakage through the wall of the bladder 30. A thinwall, between 0.012 inch and 0.016 inch thick, allows for the requiredamount of stretch while minimizing both fluid leakage and the forcerequired to pull the bladder 30 against the surrounding inner surface 20a of the tube 20.

With a thin material of the specified thickness, it is necessary toprovide a material having a high tear strength (for silicone rubber, forexample, about 125 lb/in.), so that the bladder 30 will not be damagedin use as a result of being snagged by a fingernail or rough, courseskin.

The exact shape of the bladder 30 is also very important. In earlytesting we found that when a vacuum is created behind a simple tubularbladder, the ends of the bladder always retracted first. This preventedthe bladder from retracting with a smooth surface at the center sectionwhere the appendage is placed. Instead, there would be a fold in thecenter section that would prevent the bladder from conforming to thebody appendage. The present invention employs a combination of featuresto overcome this problem.

First, two stiffener ribs 70 are placed on the inner surface 30 a of thebladder 30, parallel to each other and to the longitudinal axis of thetube 20 at diametrically opposite positions, to reduce buckling. Theribs 70 must be wide enough and thick enough to provide the stabilityneeded, but they must be narrow enough so that the stretch area of thebladder 30 is not reduced to the point that the force necessary to causethe bladder 30 to retract against the inner surface 20 a of the tube 20is too great. In addition, the height of the stiffener ribs 70 must beminimized in order to maximize the retracted diameter of the bladder 30for a given diameter of the inner surface 20 a, and to allow for theinsertion of the largest appendage of the type being targeted. The exactdimensions of the ribs 70 will therefore depend on the relative size ofthe bladder 30, the desired maximum retraction diameter of the innersurface 20 a, and the desired force required to retract the bladder 30.

Second, we have discovered that the shape and thickness of the bladder30 at the transition region 30 e between the ends 30 c and the centerportion 30 d is critical for proper bladder retraction. There must be arelatively gradual transition from the ends 30 c of the bladder 30 tothe center portion 30 d. Furthermore, the bladder thickness must beabout 30% greater within the transition region 30 e as compared to thethickness of the center portion 30 d, to increase the stiffness of thebladder 30 and decrease transition buckling upon bladder retraction. Wehave also found that the ends 30 c of the bladder 30 must overlap theends 20 b of the tube 20 by about 30% to achieve the desired stiffness,that is, to decrease buckling at the transition region 30 e between theends 30 c and the center portion 30 d upon retraction of the bladder 30.

The thickness of the bladder 30 also is greater (about 70% thicker)where the ends 30 c of the bladder 30 overlap the ends 20 b of the tube20. End caps 80 are provided at the ends 20 b of the tube 20 to coverthe ends 30 c of the bladder 30 where they overlap the ends 20 b of thetube 20.

In an embodiment in which the cuff 10 is used to make one or moreoptical measurements in addition to the volume measurement, the emitters50 a and detectors 50 b must be attached to the cuff 10 such that theyare held firmly against the appendage without allowing an air gap orcreating a ridge line that pushes against the appendage. The manner inwhich the emitters 50 a and detectors 50 b are attached must be suchthat they are secure and cannot separate from the bladder 30. In orderto make a smooth surface against the appendage with no ridge lines, theemitters 50 a and detectors 50 b are placed on the inner surface 30 a ofthe bladder 30, and the emitters 50 a and detectors 50 b respectivelyemit and detect light through the bladder 30. In particular, one of thestiffener ribs 70 includes a receiving cavity 72 dimensioned toaccommodate the emitters 50 a and detectors 50 b so that they are alwaysplaced in a consistent manner. Once the emitters 50 a and detectors 50 bhave been placed within the receiving cavity 72, an adhesivespecifically made for the bladder material is applied as an overlay toattach the emitters 50 a and detectors 50 b to the bladder 30. Theapplication and adhesion methods are critical to ensure that no air orexcess glue remain between the surface of the emitters 50 a or detectors50 b and the inner bladder surface 30 a.

To ensure consistent optical and pressure measurements, the placement ofthe appendage within the external volume 40 b must be consistent withrespect to the emitters 50 a and detectors 50 b. Furthermore, theplacement of the appendage must be in a manner to minimize anyanatomical or physiological disturbances of the appendage. In theembodiment in which the cuff 10 is used, a sensitive capacitive touchsensor is used as a finger-stop 82 to ensure proper finger positioningwithin the external volume 40 b and to minimize blanching. In addition,a gap 84 between the upper surface of the finger sensor and thecorresponding end cap 80 accommodates a large range of finger nailsizes.

Placing the emitters 50 a and detectors 50 b on the inside surface ofthe bladder 30 raises another consideration that must be addressed.Light passing though the bladder 30 from the emitters 50 a to thedetectors 50 b without passing though the appendage is called lightpiping, and is an error source that must be eliminated. To eliminatelight piping, the bladder material is tinted with appropriate pigmentsselected to absorb the specific wavelengths of light employed,effectively damping the light piping signal. The nature of the tint mustbe such that at the wavelength of light of interest is heavilyattenuated for the minimum separation distance of the source anddetector. In a preferred embodiment of the bladder-based cuff 10, theminimum distance is the distance between the illuminating source (theemitters 50 a) and the closest of the detectors 50 b. This distance isgenerally no less than 4-5 mm. The attenuation should be such todecrease the intensity of piped light to below that of the governingsignal-to-noise ratio (“SNR”) of the detecting channels. That is, thevoltage induced within the detectors 50 b due to light piping should beless than the inherit noise of the electronics and other sources.

In addition to damping the light piping signal, it is desirable to allowas much light as possible to pass from the emitters 50 a through thebladder 30 and into the appendage. As a result, the amount andcombination of pigment in the bladder 30 is critical. Acceptable tintingis therefore a function of the initial electromagnetic intensity,wavelength, bladder thickness, and distance to the nearest detector 50b. An acceptable tinting has been accomplished for wavelengths between600 and 900 nm at a concentration of 100 parts adhesive to 22 and 20parts black and blue silicone pigments, respectively.

For the most accurate optical measurements, illumination and detectionsurfaces should not produce inhomogeneities in the finger or tissuebeds. The soft and pliable surface of the bladder 30 offers an idealtissue-to-optical interface by supplying a continuous engagement surfacefor the finger and measuring system. To maintain the best engagementsurface possible, the placement of the electro-optical components (thatis, the emitters 50 a and detectors 50 b) is therefore on the side ofthe bladder 30 opposite the finger placement. However, this oppositeplacement of the emitters 50 a and detectors 50 b relative to the fingernecessitates the transmission of the light through the bladder 30 twice,once for illumination and once for detection. Thus, in addition to theoptimization of tinting substance(s) and concentration(s), it is alsonecessary to optimize the thickness of the bladder 30 between the fingerand the illumination source (that is, the emitters 50 a). In otherwords, the bladder material must be optimized to allow sufficient lightto pass therethrough into and out of the tissue to: (1) properlyilluminate the tissue and (2) properly detect the light back-scatteredfrom the tissue.

Building on the need both to attenuate unintentional traverse piping oflight between the illuminating source (emitters 50 a) and the detectors50 b, specifically selected tinting substances employed within thebladder material can provide wavelength selectivity. For example, anyambient or stray light that couples through finger or tissue bed intothe detectors 50 b that does not originate from the designatedillumination source(s) (emitters 50 a), results in measurement errors oruncertainties. Accordingly, the selection and concentration of tintingsubstances should provide wide-spectrum attenuation to minimizenon-specific light detection. Conversely, the tinting selection andconcentration may be such that a specific wavelength is selected orallowed to pass through the bladder 30 without significant attenuation,while all others are inhibited. Carefully considering thecharacteristics of the bladder material, the geometric spacing of theoptical components, and the wavelengths of the emitters 50 a, thetinting and design of the bladder 30 can be optimized to meet a widevariety of light filtering needs. These filtering needs can include, butare not limited to, high-pass, low-pass, and notch-filterconfigurations.

To accommodate appendages other than fingers and finger tips, it is onlynecessary to modify the size of the above-described cuff 10, thusmaintaining its functionality.

Modifications and variations of the above-described embodiments of thepresent invention are possible, as appreciated by those skilled in theart in light of the above teachings. It is therefore to be understoodthat, within the scope of the appended claims and their equivalents, theinvention may be practiced otherwise than as specifically described.

1. A cuff for measuring physiological parameters of an appendage,including volume or change in volume of the appendage, comprising: ahollow, rigid tube having an inner surface and opposed ends; a bladderhaving an inner surface, an outer surface, and opposed ends, the ends ofthe bladder being sealed to the ends of the tube to create an enclosedinternal volume between the inner surface of the bladder and the innersurface of the tube and an external volume defined by the outer surfaceof the bladder and surrounded by the internal volume, the bladder havinga normal, relaxed state, in which the internal volume is filled with afluid and a retracted state in which the fluid is evacuated from theinternal volume; and emitter means for emitting at least one wavelengthof light and detector means separated from the emitter means by aseparation distance for detecting the at least one wavelength of lightemitted by the emitter means, the emitter means and the detector meansbeing positioned in the enclosed internal volume for measuring volume orchange in volume of the appendage, the number of wavelengths of lightemitted by the emitter means being governed by the number ofphysiological parameters being measured other than volume or change involume, and the number of detector means being governed by the need forspatial differentiation; wherein the bladder has a sufficient wallthickness and is made from a material tinted with pigments selected suchthat the bladder material will absorb the specific wavelengths of lightemitted by the emitter means to damp light piping but also allow forsufficient transmission of light through the cuff into and out of theappendage to properly illuminate the tissue and to properly detect thelight back-scattered from the appendage.
 2. The cuff of claim 1, whereinthe bladder is made from a material tinted with pigments selected suchthat the bladder material also will achieve attenuation such as todecrease the intensity of the piped light to below the governing SNR ofthe detector means.